rtspconnection: Fix GError set over the top of a previous GError
The function fill_bytes could sometimes return a value greater than zero and in the same time set the GError.
Function read_bytes calls fill_bytes in a while loop. In the special case above it would call fill_bytes with error already set. Thus resulting in "GError set over the top of a previous GError".
Solved this by clearing GError when return value is greater than zero. Actions are taken depending on error type by caller of read_bytes. Eg. with EWOULDBLOCK gst_rtsp_source_dispatch_read will try to read the missing bytes again (GST_RTSP_EINTR)
#445 (closed) Fixes #445 (closed)